1. Field of the Invention
The present invention relates to a fixing unit which heats and melts unfixed toner, thereby fixing the toner to paper, while paper bearing a toner image is passed between a pair of heated rollers or into a nip between a heated belt and a roller, and to an image forming apparatus which comprises this fixing unit.
2. Description of the Related Art
Recently reduction of time and energy for warming up the fixing unit is required for apparatuses which fix toner using thermal energy. In respect of these requirements, a belt-type of the fixing units, which allows a decrease in the heat capacity, has been developed (see, for example, Japanese Patent Application Publication No. H6-318001). Furthermore, in recent years, an electromagnetic induction heating method (IH) which is capable of fast and highly efficient heating has been applied to the fixing unit. In order to save energy for fixing color images, the belt type of the fixing unit with electromagnetic induction heating system has been developed and image forming apparatuses including such fixing unit has been launched into the market. The belt type of the fixing unit with electromagnetic induction heating system simplifies coil design and layout, as well as facilitating to cool the coil. An electromagnetic induction system disposed on the outer side of the belt of the fixing unit (a so-called “external wrap IH”) directly heats the belt.
The fixing unit disclosed in Japanese Patent Application Publication No. 2003-107941 and Japanese Patent Publication No. 3527442 includes an external wrap IH system and does not overheat a portion of the belt with which the paper does not contact during its passage.
The fixing unit disclosed in Japanese Patent Application Publication No. 2003-107941 comprises a plurality of magnetic members which are arranged in the width direction of the paper passing through the fixing unit. At least one of the plurality of magnetic members is distanced from or moved toward an excitation coil in accordance with the width dimension of the paper passing through the fixing unit. When a magnetic member located in a position at which the paper does not pass is distanced from the excitation coil, the heating efficiency falls. Consequently, the amount of heat generated in a region where a distanced magnetic member is located is smaller than the amount of heat generated in a region where other magnetic members are located.
The fixing unit disclosed by the Japanese Patent Publication No. 3527442 comprises a conductive member which can be moved inside and outside the effective range of a magnetic field. Firstly, a conductive member is positioned outside the effective range of the magnetic field and a heating roller is heated with electromagnetic induction. If the temperature of the heating roller approaches the Curie temperature, then the conductive member moves inside the effective range of the magnetic field. A magnetic flux leaks from the heating roller to the outside of the region where the narrowest paper among the several papers which run in the image forming apparatus passes, thereby preventing excessive temperature rise. A larger conductive member is more capable of suppressing excessive temperature rise, but is not better at completely withdrawing from the effective range of the magnetic field. A small portion of the large conductive member remaining in the effective range of the magnetic field affects the magnetic field. Consequently, enlargement in surface area of the conductive member may provide undesirable effects while it may contribute to suppressing excessive temperature rise.